Film comprising a strippable sacrificial layer for reduction of surface defects in a substrate

ABSTRACT

A coextruded biaxially oriented composite film comprising a polyester substrate layer and disposed on one or both surfaces thereof a strippable sacrificial layer, wherein said strippable sacrificial layer comprises an ethylene-methacrylic acid (EMAA) copolymer.

This application is a National Stage filing of International ApplicationNo. PCT/GB2012/000240, filed 13 Mar. 2012, and claims priority of GBApplication No. 1104565.5, filed 17 Mar. 2011, the entireties of whichapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is concerned with a polyester film (particularly apolyethylene terephthalate (PET) film) having a strippable sacrificialprotective layer, and with a process for the production thereof.

BACKGROUND OF THE INVENTION

The advantageous mechanical properties, dimensional stability andoptical properties of biaxially oriented polyester films are well-known.These properties have led to the use of polyester films in electronicand opto-electronic devices including electroluminescent (EL) displaydevices (particularly organic light emitting display (OLED) devices),electrophoretic displays (e-paper), photovoltaic cells and semiconductordevices (such as organic field effect transistors, thin film transistorsand integrated circuits generally). For these and other applications, itis sometimes necessary to provide a very smooth and flat surface for thefurther processing of the polyester film, for instance to ensure theintegrity of subsequently applied coatings, such as a conductive coatingor a barrier coating, in order to avoid breaks or pin-pricks therein. Inthe manufacture of flexible electronic or opto-electronic displaydevices, for instance, a conductive layer such as indium tin oxide (ITO)may be disposed on a film substrate via a sputtering technique, andnon-uniformity in the substrate surface can cause non-uniformity and/ordiscontinuities in the conductive layer, resulting in for examplenon-uniform conductivity or pixel yield problems, depending on the typeof electronic device.

It is known to reduce defects in layers subsequently applied to asubstrate by the provision of an intermediate planarising layer, astaught in WO-03/087247-A for instance. An alternative approach is toprovide strippable sacrificial protective layers which are easilypeelable from a substrate surface, in order to protect that surface fromdamage, contamination and/or debris during storage or transport. Thesacrificial layers are then stripped from the substrate to leave a cleansurface immediately prior to the further processing or installation ofthe substrate. For instance, WO-2009/105427-A discloses a compositeoptical compensation film comprising a polyolefin substrate andstrippable layers comprising from a variety of polymeric materialsincluding other polyolefins, polyesters, and ionomers. EP-0345884-Adiscloses a coextruded strippable polyolefin or polyamide film on thesurface of a polycarbonate sheet to avoid loss of a volatile UVstabiliser in the polycarbonate, to protect the production equipmentfrom exuded amounts of the stabiliser, and to protect the polycarbonatesurface during handling and shipping. EP-0663867-A teaches the provisionof strippable sacrificial layers to a multilayer polymeric body bycoating and lamination techniques. Other disclosures of strippable skinlayers include U.S. Pat. No. 4,540,623 and U.S. Pat. No. 7,396,632. Theprior art, however, is primarily focussed on the need to protect asubstrate surface from extrinsic debris or physical damage duringstorage or transport.

It would be desirable to provide a polyester film wherein one or bothsurfaces thereof exhibit minimal or no surface defects (low intrinsicprocess defects), and high surface cleanliness (i.e. low extrinsicdebris).

SUMMARY OF THE INVENTION

According to the present invention, there is provided a coextrudedbiaxially oriented composite film comprising a polyester substrate layerand disposed on one or both surfaces thereof a strippable sacrificiallayer, wherein said strippable sacrificial layer comprises anethylene-methacrylic acid (EMAA) copolymer, and wherein the polyester isderived from: (i) one or more diol(s); (ii) one or more aromaticdicarboxylic acid(s); and (iii) optionally, one or more aliphaticdicarboxylic acid(s) of formula C_(n)H_(2n)(COOH)₂ wherein n is 2 to 8,wherein said aromatic dicarboxylic acid(s) is/are present in thepolyester in an amount of from about 80 to about 100 mole % based on thetotal amount of dicarboxylic acid components in the polyester.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a large-area metrology (LAM) image showing a pinch-pointdefect in a polyester substrate.

FIG. 1B is a LAM image showing a smooth circular defect in a polyestersubstrate.

FIG. 2 is a LAM image showing depressions in a polyester substrate,characterised by a central crater and associated raised areas around thecrater.

FIG. 3 is a LAM image showing gel-like features or streaks in apolyester substrate, characterised by globular surface features orelongated regions of raised ridges and associated shallow valleys eitherside of the ridges.

DETAILED DESCRIPTION OF THE INVENTION

The cleanliness and smoothness of the substrate surface after removal ofthe strippable sacrificial layer can be described in terms of“extrinsic” and “intrinsic” roughness. As used herein, the term“extrinsic” roughness refers to roughness resulting from air-bornedebris and/or handling damage which the substrate may suffer duringstorage and/or transport. As used herein, the term “intrinsic” roughnessrefers to roughness which is present in the substrate itself or is aresult of the process history of the film. The intrinsic roughness ofthe substrate includes any roughness induced by the presence of thestrippable sacrificial layer or its removal.

The composite film according to the invention exhibits the followingadvantages:

-   -   (i) The film is manufactured by coextrusion and hence its        production process is more economic and efficient than a        two-step process in which the sacrificial layer is applied to a        finished substrate. Thus, the strippable sacrificial layer is        not only peelable from the polyester substrate but it is also        co-extrudable with the polyester substrate.    -   (ii) The strippable sacrificial layer is reliably extrudable and        processible under the conditions conventionally used to        manufacture biaxially oriented polyester films, and is        extrudable with a uniform thickness and without MD lines. The        composite film thereby exhibits good windability and allows the        formation of a uniform roll of film, which can be a problem with        some conventional strippable sacrificial layers.    -   (iii) The strippable sacrificial layer is easily peelable from        the polyester substrate, but the interfacial adhesive strength        is not so low that it spontaneously peels apart from the        substrate during manufacture, during storage or during transport        of the composite film, which can be a problem with some        conventional strippable sacrificial layers. Furthermore, while        modulation of interfacial adhesion between the substrate and        strippable layers has conventionally been possible by using an        intermediate tie-layer therebetween, it is an advantage of this        invention that there is no need for intermediate tie-layer,        which provides economy and process efficiency in the        manufacturing process.    -   (iv) The strippable sacrificial layer is easily peelable from        the polyester substrate, especially in a roll-to-roll process,        without leaving remnants or residue of itself on the surface of        the polyester substrate or otherwise disrupting the surface of        the polyester substrate when peeled, which can be a problem with        some conventional strippable sacrificial layers. Thus, the        strippable sacrificial layer of the present invention exhibits        an easy and clean peel.    -   (v) The strippable sacrificial layer performs the required        function of protecting the polyester substrate from damage or        extrinsic debris during storage or transport until it is ready        to be used or further processed. However, the strippable        sacrificial layer must retain, or at least not significantly        degrade, the intrinsic surface smoothness of the underlying        polyester substrate, as well as other properties (such as haze)        of the substrate. A problem with many strippable sacrificial        layers is that they impart significant additional texture or        roughness or other defect to the surface of the underlying        substrate which would not have been present in the absence of        the strippable layer. For example, while the present inventors        do not intend to be bound by theory, it is believed that the        presence of gels or other particulates in the polymer matrix of        strippable sacrificial layers can induce depressions in the        underlying substrate layer which remain after removal of the        sacrificial layer. The preferred strippable sacrificial layers        of the present invention reduce such problems.    -   (vi) Moreover, the preferred strippable layers of the present        invention can advantageously reduce the intrinsic surface        defects of the film. Thus, the presence of the strippable layer        during processing can improve the surface smoothness of the        polyester substrate, when compared to the same polyester        substrate manufactured under the same conditions in the absence        of the strippable layer. Such surface defects are discussed in        more detail below and include, for instance, scratches,        pinch-point defects, smooth circular defects, gels, streaks,        flow-lines, MD-lines, die-lines and handling marks (for instance        as a result of surface imperfections in the rollers on the film        line) which can arise during manufacture of the film.        Pinch-point defects are a primary source of disruption to a        subsequently applied layer, and their reduction is a particular        objective of this invention. They have a much greater disruptive        effect to subsequently applied layers than, for example, the        smooth circular defects.

It is very surprising that the biaxially oriented polyester base layerand EMAA strippable layer of the present invention exhibit thiscombination of properties.

The biaxially oriented composite film is a self-supporting film or sheetby which is meant a film or sheet capable of independent existence inthe absence of a supporting base. According to the present invention,both the polyester substrate and the strippable layer are alsoself-supporting.

The term “polyester” as used herein includes a polyester homopolymer inits simplest form or modified, chemically and/or physically. The term“polyester” further includes copolyesters. A copolyester may be arandom, alternating or block copolyester. In a preferred embodiment, thedicarboxylic acid(s) which make up said polyester are aromaticdicarboxylic acid(s). In a preferred embodiment, the polyester comprisesonly one diol and only one dicarboxylic acid, which is preferably anaromatic dicarboxylic acid.

The aromatic dicarboxylic acid is preferably selected from terephthalicacid, isophathalic acid, phthalic acid, 1,4-, 2,5-, 2,6- or2,7-naphthalenedicarboxylic acid, and is preferably terephthalic acid or2,6-naphthalenedicarboxylic acid, preferably 2,6-naphthalenedicarboxylicacid. The diol is preferably selected from aliphatic and cycloaliphaticglycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol,neopentyl glycol and 1,4-cyclohexanedimethanol, preferably fromaliphatic glycols. Preferably the polyester contains only one glycol,preferably ethylene glycol. The aliphatic dicarboxylic acid may besuccinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid,azeleic acid or sebacic acid. Preferred homopolyesters are polyesters of2,6-naphthalenedicarboxylic acid or terephthalic acid with ethyleneglycol. It will be appreciated by those skilled in the art thatpolyesters suitable for use in the present invention arewater-insoluble.

The polyester resin is the major component of the substrate, and makesup at least 50%, preferably at least 65%, preferably at least 80%,preferably at least 90%, and preferably at least 95% by weight of thetotal weight of the substrate.

The intrinsic viscosity (IV) of the polyester from which the substrateis manufactured is typically at least about 0.58, more typically atleast about 0.60, and typically no more than about 0.70. In a preferredembodiment, a PET polyester has an IV in the range of from about 0.6 toabout 0.65, and a PEN polyester has an IV in the range of from about0.58 to about 0.68. In an alternative embodiment, the substrate can bemanufactured from a polyester with a higher intrinsic viscosity, forinstance, having an IV of at least about 0.70, and in a furtherembodiment at least about 0.80, and typically no more than 0.90.

The polyester is obtainable from said dicarboxylic acid(s) or theirlower alkyl (up to 6 carbon atoms) diesters with one or more diols.Formation of the polyester is conveniently effected in a known manner bycondensation or ester interchange, generally at temperatures up to about295° C. In one embodiment, solid state polymerisation may be used toincrease the intrinsic viscosity to the desired value, usingconventional techniques well-known in the art, for instance using afluidised bed such as a nitrogen fluidised bed or a vacuum fluidised bedusing a rotary vacuum drier.

In one embodiment, the substrate may further comprise a UV-absorber. TheUV-absorber has an extinction coefficient much higher than that of thepolyester such that most of the incident UV light is absorbed by theUV-absorber rather than by the polyester. The UV-absorber generallydissipates the absorbed energy as heat, thereby avoiding degradation ofthe polymer chain, and improving the stability of the polyester to UVlight. Typically, the UV-absorber is an organic UV-absorber, andsuitable examples include those disclosed in Encyclopaedia of ChemicalTechnology, Kirk-Othmer, Third Edition, John Wiley & Sons, Volume 23,Pages 615 to 627. Particular examples of UV-absorbers includebenzophenones, benzotriazoles (U.S. Pat. No. 4,684,679, U.S. Pat. No.4,812,498 and U.S. Pat. No. 4,681,905), benzoxazinones (U.S. Pat. No.4,446,262, U.S. Pat. No. 5,251,064 and U.S. Pat. No. 5,264,539) andtriazines (U.S. Pat. No. 3,244,708, U.S. Pat. No. 3,843,371, U.S. Pat.No. 4,619,956, U.S. Pat. No. 5,288,778 and WO 94/05645). The UV-absorbermay be incorporated into the film according to one of the methodsdescribed herein. In one embodiment, the UV-absorber may be chemicallyincorporated in the polyester chain. EP-A-0006686, EP-A-0031202,EP-A-0031203 and EP-A-0076582, for example, describe the incorporationof a benzophenone into the polyester. The specific teaching of theaforementioned documents regarding UV-absorbers is incorporated hereinby reference. In a particularly preferred embodiment, improvedUV-stability in the present invention is provided by triazines, morepreferably hydroxyphenyltriazines, and particularlyhydroxyphenyltriazine compounds of formula (II):

wherein R is hydrogen, C₁-C₁₈ alkyl, C₂-C₆ alkyl substituted by halogenor by C₁-C₁₂ alkoxy, or is benzyl and R¹ is hydrogen or methyl. R ispreferably C₁-C₁₂ alkyl or benzyl, more preferably C₃-C₆ alkyl, andparticularly hexyl. R¹ is preferably hydrogen. An especially preferredUV-absorber is 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(hexyl)oxy-phenol,which is commercially available as Tinuvin™ 1577 FF from Ciba-Additives.

The amount of UV-absorber is preferably in the range from 0.1% to 10%,more preferably 0.2% to 7%, more preferably 0.6% to 4%, particularly0.8% to 2%, and especially 0.9% to 1.2% by weight, relative to the totalweight of the substrate.

The substrate may also comprise an anti-oxidant, which may be present inaddition to, or instead of, said UV-stabiliser. A range of antioxidantsmay be used, such as antioxidants which work by trapping radicals or bydecomposing peroxide. Suitable radical-trapping antioxidants includehindered phenols, secondary aromatic amines and hindered amines, such asTinuvin™ 770 (Ciba-Geigy). Suitable peroxide-decomposing antioxidantsinclude trivalent phosphorous compounds, such as phosphonites,phosphites (e.g. triphenyl phosphate and trialkylphosphites) andthiosynergists (e.g. esters of thiodipropionic acid, such as dilaurylthiodipropionate). Hindered phenol antioxidants are preferred. Apreferred hindered phenol is tetrakis-(methylene3-(4′-hydroxy-3′,5′-di-t-butylphenyl propionate) methane, which iscommercially available as Irganox™ 1010 (Ciba-Geigy). Other suitablecommercially available hindered phenols include Irganox™ 1035, 1076,1098 and 1330 (Ciba-Geigy), Santanox™ R (Monsanto), Cyanox™ antioxidants(American Cyanamid) and Goodrite™ antioxidants (BF Goodrich). Theconcentration of antioxidant present in the substrate is preferably inthe range from 50 ppm to 5000 ppm of the polyester, more preferably inthe range from 300 ppm to 1500 ppm, particularly in the range from 400ppm to 1200 ppm, and especially in the range from 450 ppm to 600 ppm. Amixture of more than one antioxidant may be used, in which case thetotal concentration thereof is preferably within the aforementionedranges. Incorporation of the antioxidant into the substrate may beeffected by conventional techniques, and preferably by mixing with themonomeric reactants from which the polyester is derived, particularly atthe end of the direct esterification or ester exchange reaction, priorto polycondensation.

The polyester substrate may further comprise any other additiveconventionally employed in the manufacture of polyester films. Thus,agents such as cross-linking agents, dyes, fillers, pigments, voidingagents, lubricants, radical scavengers, thermal stabilisers, flameretardants and inhibitors, anti-blocking agents, surface active agents,slip aids, gloss improvers, prodegradents, viscosity modifiers anddispersion stabilisers may be incorporated as appropriate. Suchcomponents may be introduced into the polymer in a conventional manner.For example, by mixing with the monomeric reactants from which thefilm-forming polymer is derived, or the components may be mixed with thepolymer by tumble or dry blending or by compounding in an extruder,followed by cooling and, usually, comminution into granules or chips.Masterbatching technology may also be employed.

The substrate may, in particular, comprise a particulate filler whichcan improve handling and windability during manufacture, and can be usedto modulate optical properties. The particulate filler may, for example,be a particulate inorganic filler (e.g. metal or metalloid oxides, suchas alumina, titania, talc and silica (especially precipitated ordiatomaceous silica and silica gels), calcined china clay and alkalinemetal salts, such as the carbonates and sulphates of calcium andbarium). Any inorganic filler present should be finely-divided, and thevolume distributed median particle diameter (equivalent sphericaldiameter corresponding to 50% of the volume of all the particles, readon the cumulative distribution curve relating volume % to the diameterof the particles—often referred to as the “D(v,0.5)” value) thereof ispreferably in the range from 0.01 to 5 μm, more preferably 0.05 to 1.5μm, and particularly 0.15 to 1.2 μm. Preferably at least 90%, morepreferably at least 95% by volume of the inorganic filler particles arewithin the range of the volume distributed median particle diameter ±0.8μm, and particularly ±0.5 μm. Particle size of the filler particles maybe measured by electron microscope, coulter counter, sedimentationanalysis and static or dynamic light scattering. Techniques based onlaser light diffraction are preferred.

Typically, the polyester substrate is optically clear, since themajority of end-uses of the substrate demand good aesthetic appearance.Preferably the substrate has a % of scattered visible light (haze) of nomore than 15%, preferably no more than 10%, preferably no more than 6%,more preferably no more than 3.5%, more preferably no more than 1.5%,and particularly no more than 1.0%, and/or a total luminous transmission(TLT) for light in the visible region (400 nm to 700 nm) of at least80%, preferably at least 85%, more preferably at least about 90%. Inthis embodiment, any filler in the substrate is typically present inonly small amounts, generally not exceeding 0.5% and preferably lessthan 0.2% by weight of a layer, and the filler is preferably silica. Inthis embodiment, the windability of the film (i.e. the absence ofblocking or sticking when the film is wound up into a roll) is improved,without an unacceptable reduction in haze or other optical properties.

In an alternative embodiment, the substrate is opaque. An opaque filmpreferably exhibits a Transmission Optical Density (TOD) of at least0.4, preferably at least 0.5, preferably at least 0.6, preferably atleast 0.7, preferably at least 1.0 and preferably at least 1.5, and inone embodiment preferably at least 2.0, preferably at least 3.0, andpreferably at least 4.0. An opaque film may be pigmented as required,and in one embodiment, the substrate is white, grey or black. Suitablewhitening agents include a particulate inorganic filler such as thosereferred to hereinabove, an incompatible resin filler, or a mixture oftwo or more such fillers. Suitable opacifying agents include carbonblack, or a metallic filler such as aluminium powder, as is known in theart.

The intrinsic viscosity of the polyester substrate is typically lowerthan that of the polyester from which it is manufactured, and the IVdrop during the preparation of a biaxially oriented polyester film froma polyester raw material can be as much as 0.15, particularly forpolyesters having a relatively high initial IV. Typically, however, theIV drop is less than about 0.06. In one embodiment, the IV of thepolyester substrate is at least about 0.52, preferably at least about0.60 and typically no more than about 0.70. Typically, a PET substratehas an IV in the range of from about 0.57 to about 0.65, and preferablyat least about 0.60. A PEN substrate has an IV in the range of fromabout 0.52 to about 0.68, and preferably at least about 0.60. In analternative embodiment, the polyester substrate has a higher intrinsicviscosity, for instance, having an IV of at least about 0.70, preferablyat least about 0.75, and typically no more than about 0.80.

The polyester substrate preferably exhibits a low shrinkage, preferablyno more than 3% at 150° C. over 30 minutes, preferably no more than 2%,preferably no more than 1.5%, and preferably no more than 1.0%,particularly in the machine (longitudinal dimension) of the film, andpreferably in both dimensions of the substrate (i.e. the longitudinaland transverse dimensions). The substrate should not undergounacceptable dimensional distortion, such as curl, when subjected tosubsequent processing conditions, for instance processing involvingelevated temperature (such as sputtering to deposit a subsequent layer),which may be used for instance in the manufacture of electronic displaydevices.

The strippable layer comprises (and suitably consists, or consistsessentially, of) an ethylene-methacrylic acid (EMAA) copolymer. In apreferred embodiment, the methacrylic acid is present in the copolymerin the range of from about 2 to about 15 wt % of the copolymer, morepreferably in the range of from about 2 to about 10 wt %, and preferablyin the range of from about 7 to about 10 wt %. In an alternativeembodiment, the methacrylic acid is present in the copolymer in therange of from about 2 to less than about 7 wt %. The copolymers arepreferably branched and preferably random. Suitable EMAA copolymersinclude Nucrel® resins (DuPont), particularly Nucrel® grades 0411HS and0908HS. Optionally, the copolymer has been partially or completelyreacted with metallic salts, enabling the formation of ionic cross-linksbetween the acid groups within a chain, or between neighbouring chains.Such copolymers are known as ionomers, defined herein as a polymer madeup primarily of non-polar repeat units with a minor proportion(typically no more than about 15 wt %) of metal salt-containing units ofmethacrylic acid. Preferred ionomers are the copolymers of ethylene andmethacrylic acid partially or completely neutralised with alkali metalsor zinc. Suitable commercially available compounds include Surlyn®resins (DuPont), particularly grades 1605 and 1652. The metal cation istypically selected from alkali metals such as lithium and sodium. Zincor magnesium may also be used. Typically, the metal cation is present atno more than about 15 mol %. Preferably, the EMAA copolymer is not anionomer and does not contain metal ions.

In a preferred embodiment, the melting temperature of the EMAA copolymeris at least about 90° C., and preferably no more than about 250° C.,preferably no more than about 200° C., preferably no more than about150° C., more preferably no more than about 120° C. In a furtherpreferred embodiment, the VICAT softening point of the EMAA copolymer isat least about 60° C., and typically in the range from about 60° C. toabout 110° C., more typically in the range from about 70° C. to about100° C.

Formation of the composite film is effected by conventional co-extrusiontechniques well-known in the art. In general terms the process comprisesthe steps of extruding layers of molten polymer at a temperature withinthe range of from about 280 to about 300° C., quenching the extrudateand orienting the quenched extrudate. Orientation may be effected by anyprocess known in the art for producing an oriented film, for example atubular or flat film process. Biaxial orientation is effected by drawingin two mutually perpendicular directions in the plane of the film toachieve a satisfactory combination of mechanical and physicalproperties. In a tubular process, simultaneous biaxial orientation maybe effected by extruding a thermoplastics polymer tube which issubsequently quenched, reheated and then expanded by internal gaspressure to induce transverse orientation, and withdrawn at a rate whichwill induce longitudinal orientation. In the preferred flat filmprocess, the film-forming polymer is extruded through a slot die andrapidly quenched upon a chilled casting drum to ensure that thesubstrate polyester is quenched to the amorphous state. Orientation isthen effected by stretching the quenched extrudate in at least onedirection at a temperature above the glass transition temperature of thesubstrate polyester. Sequential orientation may be effected bystretching a flat, quenched extrudate firstly in one direction, usuallythe longitudinal direction, i.e. the forward direction through the filmstretching machine, and then in the transverse direction. Forwardstretching of the extrudate is conveniently effected over a set ofrotating rolls or between two pairs of nip rolls, transverse stretchingthen being effected in a stenter apparatus. Stretching is generallyeffected so that the dimension of the oriented film is from 2 to 5, morepreferably 2.5 to 4.5 times its original dimension in the or eachdirection of stretching. Typically, stretching is effected attemperatures higher than the T_(g) of the polyester, preferably about15° C. higher than the T_(g). Greater draw ratios (for example, up toabout 8 times) may be used if orientation in only one direction isrequired. It is not necessary to stretch equally in the machine andtransverse directions although this is preferred if balanced propertiesare desired.

A stretched film may be, and preferably is, dimensionally stabilised byheat-setting under dimensional support at a temperature above the glasstransition temperature of the substrate polyester but below the meltingtemperature thereof, to induce the desired crystallisation of thepolyester. During the heat-setting, a small amount of dimensionalrelaxation may be performed in the transverse direction (TD) by aprocedure known as “toe-in”. Toe-in can involve dimensional shrinkage ofthe order 2 to 4% but an analogous dimensional relaxation in the processor machine direction (MD) is difficult to achieve since low linetensions are required and film control and winding becomes problematic.The actual heat-set temperature and time will vary depending on thecomposition of the film and its desired final thermal shrinkage butshould not be selected so as to substantially degrade the toughnessproperties of the film such as tear resistance. Within theseconstraints, a heat set temperature of about 180 to 245° C. is generallydesirable. After heat-setting the film is typically quenched rapidly inorder induce the desired crystallinity of the substrate polyester.

In one embodiment, the film may be further stabilized through use of anon-line relaxation stage. Alternatively the relaxation treatment can beperformed off-line. In this additional step, the film is heated at atemperature lower than that of the heat-setting stage, and with a muchreduced MD and TD tension. The tension experienced by the film is a lowtension and typically less than 5 kg/m, preferably less than 3.5 kg/m,more preferably in the range of from 1 to about 2.5 kg/m, and typicallyin the range of 1.5 to 2 kg/m of film width. For a relaxation processwhich controls the film speed, the reduction in film speed (andtherefore the strain relaxation) is typically in the range 0 to 2.5%,preferably 0.5 to 2.0%. There is no increase in the transverse dimensionof the film during the heat-stabilisation step. The temperature to beused for the heat stabilisation step can vary depending on the desiredcombination of properties from the final film, with a higher temperaturegiving better, i.e. lower, residual shrinkage properties. A temperatureof 135 to 250° C. is generally desirable, preferably 150 to 230° C.,more preferably 170 to 200° C. The duration of heating will depend onthe temperature used but is typically in the range of 10 to 40 seconds,with a duration of 20 to 30 seconds being preferred. This heatstabilisation process can be carried out by a variety of methods,including flat and vertical configurations and either “off-line” as aseparate process step or “in-line” as a continuation of the filmmanufacturing process. Film thus processed will exhibit a smallerthermal shrinkage than that produced in the absence of such postheat-setting relaxation.

Co-extrusion is effected either by simultaneous coextrusion of therespective film-forming layers through independent orifices of amulti-orifice die, and thereafter uniting the still molten layers or,preferably, by single-channel coextrusion in which molten streams of therespective polymers are first united within a channel leading to a diemanifold, and thereafter extruded together from the die orifice underconditions of streamline flow without intermixing thereby to produce amulti-layer film, which is oriented and heat-set as hereinbeforedescribed. It will therefore be appreciated by those skilled in the artthat the strippable sacrificial layer is disposed directly on one orboth surface(s) of said polyester substrate, i.e. without anyintermediate layer.

The thickness of the composite film is preferably in the range of fromabout 5 to about 750 μm, and more preferably no more than about 500 μm,and typically between about 12 μm and 250 μm. The thickness of thesubstrate layer is preferably in the range of from about 5 to about 500μm, and typically between about 12 μm and 300 μm. The thickness of thestrippable layer is preferably in the range of from about 2 to about 200μm, and typically no more than about 100 μm, and typically between about5 μm and 50 μm, and in preferred embodiment between about 5 and 25 μm.It is preferred that the substrate layer makes up greater than 50%,preferably at least 60%, preferably at least 70%, and preferably atleast 80% of the total thickness of the composite film, but typically nomore than about 95% of the total thickness. In one embodiment, thesubstrate layer makes up from about 75 to about 95% of the totalthickness of the composite film. The strippable layer typically ends upas waste film and becomes uneconomical if it is too thick, but thestrippable layer must have a sufficient thickness and mechanicalstrength to allow an easy and clean peel.

The adhesive strength of the strippable sacrificial layer to thepolyester substrate is such that the peel force is preferably in therange of from about 5 to about 250 gF (grams-Force), preferably at leastabout 10 gF, preferably at least about 20 gF, preferably at least about35 gF, typically at least about 50 gF, and typically no more than about200 gF, measured as described herein.

In the present invention, the intrinsic surface roughness of thepolyester substrate after removal of the strippable layer is primarilyanalysed by two methods.

The first method analyses the intrinsic “micro-roughness”, i.e. thebackground surface roughness between major surface defects, measured asdescribed herein and preferably characterised by the conventionalparameters of Ra and/or Rq. Preferably, the polyester substrate exhibitsan Ra value, of less than 10 nm, preferably less than 7 nm, preferablyless than 5 nm, preferably less than 2 nm, and preferably less than 1nm.

The second method analyses the intrinsic “macro-roughness” usinglarge-area metrology (LAM) which analyses the major intrinsic surfacedefects. The major intrinsic surface defects in the polyester substratecan be categorised as defects (1) to (3) as follows:

-   -   (1) Defects caused by inclusions within the polyester substrate,        which can be caused by the presence of, for instance, fillers,        crystallites, degradants and gels (typically regions of        intractable polymer (for instance, cross-linked, branched or        degraded polymer) having a molecular weight or rheology        different to the bulk polymer matrix), and which can be        categorised as “pinch-point” defects and “smooth circular”        defects:        -   (a) Pinch-point defects are characterised by a crater either            side of a central peak (see FIG. 1A). It is believed that            such defects are caused by inclusions which reside            relatively near the surface of the substrate, which generate            greater localised stress/strain regions during the            stretching steps of film manufacture.        -   (b) Smooth circular defects are characterised by a central            peak with no crater (see FIG. 1B). It is believed that such            defects are caused by inclusions which reside relatively            deeper within the substrate    -   (2) Depressions are characterised by a central crater, sometimes        associated with raised areas around the crater (see FIG. 2). The        inventors believe such defects are caused primarily by        imperfections (such as gels or gel-like features) in the        strippable sacrificial layer.    -   (3) Gel-like features or streaks are characterised by globular        surface features or elongated regions of raised ridges sometimes        associated with shallow valleys either side of the ridge (see        FIG. 3). The inventors believe such defects result from        extrusion events, such as die-lip edge flow disturbances and        extruded degraded polymer.

Upon removal of the strippable sacrificial layer, the polyestersubstrate preferably exhibits one or more of the following intrinsicsurface roughness properties, particularly wherein intrinsic surfaceroughness is evaluated in respect of the major intrinsic surface defectsdefined above as (1a), (1b), (2) and (3), and wherein the values of peakheight are expressed as the Rp parameter defined herein and the valuesof crater depth are expressed as the Rv parameter defined herein andmeasured by phase shift interferometry (PSI) or vertical scanninginterferometry (VSI) in the LAM technique as described hereinbelow:

-   -   (i) The number (N_(DT)) of all defects with a vertical        amplitude, orthogonal to the film plane (i.e. peaks and        troughs), of greater than about 0.25 μm and less than about 30        μm above and/or below the mean surface (as defined herein) is no        more than 1000, preferably no more than 750, preferably no more        than 500, preferably no more than about 400, preferably no more        than about 300, preferably no more than 200, preferably no more        than 100, preferably no more than 75, preferably no more than        50, and preferably no more than 25, per 31×33 cm area of film        surface.    -   (ii) The number (N_(PP)) of pinch-point peaks (1a) with a peak        height of greater than about 0.25 μm and less than about 30 μm        is no more than 100, preferably no more than 80, preferably no        more than 70, preferably no more than 60, preferably no more        than 50, preferably no more than 40, preferably no more than 30,        and preferably no more than 20, per 31×33 cm area of film        surface.    -   (iii) The number (N_(GS)) of gel-like features or streaks (3)        with a peak height of greater than about 0.25 μm and less than        about 30 μm is no more than 10, preferably no more than 5,        preferably no more than 2, and preferably zero, per 31×33 cm        area of film surface.    -   (iv) The improvement (Δ-N_(DT)) in the parameter (N_(DT)),        relative to a control polyester substrate manufactured without        the strippable sacrificial layer, wherein Δ-N_(DT) is defined as        [N _(DT) of control substrate]/[N _(DT) of stripped inventive        substrate]        -   is preferably at least 2, preferably at least 4, preferably            at least 6, preferably at least 7, preferably at least 10.    -   (v) The improvement (Δ-N_(PP)) in the parameter (N_(PP)),        relative to a control polyester substrate manufactured without        the strippable sacrificial layer, wherein Δ-N_(PP) is defined        as:        [N _(PP) of control substrate]/[N _(PP) of stripped inventive        substrate]        -   is at least 2, preferably at least 4, preferably at least 6,            preferably at least 7, preferably at least 10.    -   (vi) The improvement (Δ-N_(GS)) in the parameter (N_(GS)),        relative to a control polyester substrate manufactured without        the strippable sacrificial layer, wherein Δ-N_(GS) is defined        as:        [N _(GS) of control substrate]/[N _(GS) of stripped inventive        substrate]        -   is at least 2, preferably at least 4, preferably at least 6,            preferably at least 7, preferably at least 10.

If desired, the techniques described herein to measure the intrinsicmacro-roughness can also be used to measure the lateral dimensions ofthe defects attributable to extrinsic roughness. Such extrinsic defectsare defined herein as irregularly shaped features having positivetopography (i.e. features which are above the mean surface level of thefilm) with substantially no negative topography (i.e. features which arebelow the mean surface level of the film). If desired, the film can becharacterised in terms of the number (N_(E)) of extrinsic defects (asdefined above) with a minimum lateral dimension of greater than 7.14 μm,per 31×33 cm area of film surface, and measured by single frameinterferometry (SFI) in the LAM technique as described hereinbelow.

The composite film of the present invention may be advantageously usedin any application which requires a high-quality defect-free polyestersubstrate surface exhibiting high cleanliness and high smoothness. Thus,the composite film may be advantageously used in to provide a cleanpolyester substrate suitable for use in the manufacture of electronic oropto-electronic devices, such as electroluminescent (EL) display devices(particularly organic light emitting display (OLED) devices),electrophoretic displays (e-paper), photovoltaic (PV) cells andsemiconductor devices (such as organic field effect transistors, thinfilm transistors and integrated circuits generally), particularlyflexible such devices. Other applications include the provision ofoptical films, medical devices and decorative films.

A subsequently applied layer may be a barrier layer, i.e. a layer whichprovides high resistance to gas and solvent permeation, which istypically applied in a sputtering process at elevated temperatures. Abarrier layer may be organic or inorganic, should exhibit good affinityfor the layer deposited thereupon, and be capable of forming a smoothsurface. Materials which are suitable for use to form a barrier layerare disclosed, for instance, in U.S. Pat. No. 6,198,217.

A subsequently applied layer may be a conductive layer, which is oftenapplied in a sputtering process at elevated temperatures.

According to a further aspect of the invention, there is provided theuse of a layer comprising an ethylene-methacrylic acid (EMAA) copolymeras a strippable sacrificial layer in a coextruded biaxially orientedcomposite film further comprising a polyester substrate layer, asdescribed herein, wherein said EMAA layer is disposed on one or bothsurfaces of said polyester substrate layer.

According to a further aspect of the invention, there is provided theuse of a layer comprising an ethylene-methacrylic acid (EMAA) copolymeras a strippable sacrificial layer in a coextruded biaxially orientedcomposite film further comprising a polyester substrate layer, asdescribed herein, wherein said EMAA layer is disposed on one or bothsurfaces of said polyester substrate layer, for the purposes of:

-   -   (i) protecting a surface of said polyester substrate layer from        damage and/or contamination and/or debris during transport        and/or storage; and/or    -   (ii) reducing the intrinsic surface defects of said polyester        substrate, relative to the same polyester substrate manufactured        under the same conditions in the absence of said strippable        sacrificial layer.

According to a further aspect of the invention, there is provided amethod of protecting a surface of a substrate from damage and/orcontamination and/or debris during transport and/or storage, said methodcomprising the steps of:

(i) providing a coextruded biaxially oriented composite film comprisinga polyester substrate layer and disposed on one or both surfaces thereofa strippable sacrificial layer comprising an ethylene-methacrylic acid(EMAA) copolymer as defined herein; and

(ii) removing said strippable sacrificial layer from said substrateprior to use or further processing of said substrate.

According to a further aspect of the invention, there is provided amethod of reducing defects in a functional layer (particularly aconductive or barrier layer) applied to a substrate, said methodcomprising the steps of:

(i) providing a coextruded biaxially oriented composite film comprisinga polyester substrate layer and disposed on one or both surfaces thereofa strippable sacrificial layer comprising an ethylene-methacrylic acid(EMAA) copolymer as defined herein;

(ii) removing said strippable sacrificial layer from said substrate; and

(iii) applying said functional layer to said substrate.

According to a further aspect of the invention, there is provided anelectronic or opto-electronic device, optical film, medical device ordecorative film derived from the coextruded biaxially oriented compositefilm defined herein from which said strippable sacrificial layer hasbeen removed.

According to a further aspect of the invention, there is provided anelectronic or opto-electronic device comprising a polyester substratelayer, wherein said polyester substrate layer is derived from acoextruded biaxially oriented composite film comprising a polyestersubstrate layer and disposed on one or both surfaces thereof astrippable sacrificial layer comprising an ethylene-methacrylic acid(EMAA) copolymer as defined herein, wherein said strippable sacrificiallayer has been removed from said composite film prior to or duringincorporation into or manufacture of said electronic or opto-electronicdevice.

According to a further aspect of the invention, there is provided amethod of manufacture of an electronic or opto-electronic device,optical film, medical device or decorative film, said method comprisingthe steps of:

(i) providing a coextruded biaxially oriented composite film comprisinga polyester substrate layer and disposed on one or both surfaces thereofa strippable sacrificial layer comprising an ethylene-methacrylic acid(EMAA) copolymer as defined herein; and

(ii) removing said strippable sacrificial layer from said substrateprior to the use of said substrate in said electronic or opto-electronicdevice or medical device, or in or as said optical or decorative film.

The electronic or opto-electronic device may further comprise a barrieror conductive layer disposed on said polyester substrate. Of particularinterest are electronic or opto-electronic devices selected fromelectroluminescent (EL) display devices (particularly organic lightemitting display (OLED) devices), electrophoretic displays (e-paper),photovoltaic (PV) cells and semiconductor devices (such as organic fieldeffect transistors, thin film transistors and integrated circuitsgenerally), particularly flexible such devices.

Property Measurement

The following analyses were used to characterize the films describedherein:

-   (i) Clarity is evaluated by measuring total luminance transmission    (TLT) and haze (% of scattered transmitted visible light) through    the total thickness of the film using an M57D spherical hazemeter    (Diffusion Systems) according to the standard test method ASTM    D1003.-   (ii) Transmission Optical Density (TOD) is measured using a Macbeth    Densitometer TR 927 (obtained from Dent and Woods Ltd, Basingstoke,    UK) in transmission mode.-   (iii) Intrinsic viscosity (in units of dL/g) is measured by solution    viscometry in accordance with ASTM D5225-98(2003) on a Viscotek™    Y-501C Relative Viscometer (see, for instance, Hitchcock, Hammons &    Yau in American Laboratory (August 1994) “The dual-capillary method    for modern-day viscometry”) by using a 0.5% by weight solution of    polyester in o-chlorophenol at 25° C. and using the Billmeyer    single-point method to calculate intrinsic viscosity:    η=0.25η_(red)+0.75(ln η_(rel))/c    -   wherein:    -   η=the intrinsic viscosity (in dL/g),    -   η_(rel)=the relative viscosity,    -   c=the concentration (in g/dL), &    -   η_(red)=reduced viscosity (in dL/g), which is equivalent to        (η_(re1)−1)/c (also expressed as η_(sp)/c where η_(sp) is the        specific viscosity).-   (iv) Thermal shrinkage is assessed for film samples of dimensions    200 mm×10 mm which were cut in specific directions relative to the    machine and transverse directions of the film and marked for visual    measurement. The longer dimension of the sample (i.e. the 200 mm    dimension) corresponds to the film direction for which shrinkage is    being tested, i.e. for the assessment of shrinkage in the machine    direction, the 200 mm dimension of the test sample is oriented along    the machine direction of the film. After heating the specimen to the    predetermined temperature of 150° C. (by placing in a heated oven at    that temperature) and holding for an interval of 30 minutes, it was    cooled to room temperature and its dimensions re-measured manually.    The thermal shrinkage was calculated and expressed as a percentage    of the original length.-   (v) Melt Flow Index (MFI) is measured herein according to ASTM D1238    or ISO-1133, depending on the polymer used. The EMAA copolymers used    in the present invention are analysed in accordance with ASTM D1238    at a temperature of 190° C. and a mass of 2.16 kg. Suitable MFI    ranges for the EMAA copolymers used in the present invention are in    the range from about 0.5 to about 50 g/10 min, preferably from about    1 to about 25 g/10 min, typically from about 2 to about 20 g/10 min,    and more typically from about 2 to about 15 g/10 min.-   (vi) Layer thickness is measured by Mercer 122D gauge.-   (vii) MD lines are localised high spots or circumferential bands on    reels caused by poor film thickness profile and/or reel buckling.    Die lines are straight lines in the machine direction that remain in    the same location on the melt during filming. Flow lines are lines    in the machine or transverse direction that do not remain in the    same location on the melt during filming; they are thought to be    caused by the transient or migratory presence of polymeric spherical    gels (cross-linked) which cause a disturbance in the polymer    curtain. The presence of each of these defects in the film was    assessed qualitatively by visual inspection by the naked eye (i.e.    without a microscope).-   (viii) Scratches are low amplitude (typically up to about 1000 nm    deep and up to about 1000 nm wide) elongated depressions in the    film. They are thought to result from imperfections in the die and    rollers used in film manufacture, or from film handling. Scratches    are classified herein as intrinsic surface defects, and their    presence in the film was assessed qualitatively by optical    microscopy (at 2.5× magnification). Of course, extrinsic surface    roughness resulting from handling damage during storage or transport    can also include the scratching of the film surface, but such    defects are not measured herein.-   (ix) Peelability is initially assessed using a hand peel test, in    which the peelable film is cut with scissors to aid delamination,    and the peelable layer is then pulled by hand away from the    substrate. This crude test provides a low-cost preliminary    indication of the suitability of the film for further analysis and    investigation. The grading of the films was as follows:    -   Grade 1: strippable layer removable in its entirety, with no        sign of residue from the strippable layer remaining on the        substrate.    -   Grade 2: strippable layer removable in its entirety, with spots        of residue visible on the substrate.    -   Grade 3: strippable layer fractures when peeled.    -   Grade 4: strippable layer not possible to remove from substrate-   (x) Peel force is measured on an SST-3 Seal Strength Tester (RDM    Test Equipment) as follows. 10 mm wide strips of film are cut from    the web using a thick film tool. If the peelable layer is    well-adhered a piece of adhesive tape (Tesa 4104) is used to lift    the peelable layer from the PET substrate. The peelable layer is    then attached to double-sided tape on the upper jaw of the    equipment, and the PET substrate is attached to double-sided tape on    the lower jaw. The reading is set to zero before the jaws are moved    apart, and the jaws then moved apart at 240 mm/min. The peak value    of the force recorded to separate the layers is recorded (grams    Force). The results are averaged from three measurements. The    apparatus is reset between samples to reset the peak,-   (xi) The “micro-roughness” of the substrate surface in fields of    view (defined below) selected to be remote from any major surface    defects is characterised using conventional non-contacting,    white-light, phase-shifting interferometry techniques, which are    well-known in the art, using a Wyko NT3300 surface profiler using a    light source of wavelength 604 nm. With reference to the WYKO    Surface Profiler Technical Reference Manual (Veeco Process    Metrology, Arizona, US; June 1998; the disclosure of which is    incorporated herein by reference), the characterising data    obtainable using the technique include:    -   Averaging Parameter—Roughness Average (Ra): the arithmetic        average of the absolute values of the measured height deviations        within the evaluation area and measured from the mean surface.    -   Averaging Parameter—Root Mean Square Roughness (Rq): the root        mean square average of the measured height deviations within the        evaluation area and measured from the mean surface.    -   Peak-to-Valley value (PV₉₅): this parameter may be obtained from        the frequency distribution of positive and negative surface        heights as a function of surface height referenced to the mean        surface plane. The value PV₉₅ is the peak-to-valley height        difference which envelops 95% of the peak-to-valley surface        height data in the distribution curve by omitting the highest        and lowest 2.5% of datapoints. The PV₉₅ parameter provides a        statistically significant measure of the overall peak-to-valley        spread of surface heights.    -   The roughness parameters and peak heights are measured relative        to the average level of the sample surface area, or “mean        surface”, in accordance with conventional techniques. (A        polymeric film surface may not be perfectly flat, and often has        gentle undulations across its surface. The mean surface is a        plane that runs centrally through undulations and surface height        departures, dividing the profile such that there are equal        volumes above and below the mean surface.) The surface profile        analysis is conducted by scanning discrete regions of the film        surface (between and remote from major defects) within the        “field of view” of the surface profiler instrument, which is the        area scanned in a single measurement. A film sample may be        analysed using a discrete field of view, or by scanning        successive fields of view to form an array. The analyses        conducted herein utilised the full resolution of the Wyko NT3300        surface profiler, in which each field of view comprises 480×736        pixels. For the measurement of Ra and Rq, the resolution was        enhanced using an objective lens having a 50-times        magnification. The resultant field of view has dimensions of 90        μm×120 μm, with a pixel size of 0.163 μm. The results of five        successive scans over the same portion of the surface area are        combined to give an average value. The measurements were        conducted using a modulation threshold (a user-determined        parameter based on the minimal acceptable signal-to-noise ratio)        of 10%, i.e. data points below the threshold are identified as        unreliable,-   (xii) The macro-roughness of the substrate surface was analysed by    large-area metrology (LAM) using a Wyko SSP9910 Single Frame    Interferometer, also equipped with both PSI and VSI capability, in    order to arrive at values for the parameters of Maximum Profile Peak    Height (Rp) and Maximum Profile Crater Depth (Rv), defined as the    height (or depth) of the highest peak (or crater/trough) in the    evaluation area, as measured from the mean surface. The measurement    area of the film was 31×33 cm.    -   The first step of the technique is to conduct measurements in        the SFI (Single Frame Interferometry) mode to produce a low        magnification (×2.5) map in order to determine the location of        major surface defects in the film area studied. In the SFI mode,        the modulation threshold was set at 1%, and the cut-off        threshold (another user-determined parameter selected to define        the minimum vertical amplitude constituting a major surface        defect) was set at 0.25 μm above the mean surface. The skilled        man will appreciate that for some laterally smaller defects the        pixel size is large in comparison with the peak area in SFI        mode, and so at this low magnification the measured peak height        measured (which is averaged over the pixel) may be weighted        downwards as a result of the larger pixel area. Thus, intrinsic        defects in SFI mode are defined as those having a peak height of        at least 0.25 μm and spanning greater than 2 adjoining pixels (1        pixel=3.57 μm); and extrinsic defects were considered as those        spanning at least 3 adjoining but not necessarily co-linear        pixels (7.14 μm in at least one lateral dimension). Intrinsic        and extrinsic defects were differentiated herein according to        the reflectance profile (extrinsic defects exhibit a reflectance        profile which is different from the polyester matrix and        typically exhibit a lower reflectance). Intrinsic and extrinsic        defects may also be differentiated according to the lateral        profile of the defect. The first step in the technique produces        (x,y)-coordinates of all the user-defined defects in the film        surface.    -   The second step of the technique is to re-examine the film        surface using the same equipment in either phase shifting        interferometry (PSI) mode or vertical scanning interferometry        mode (VSI) to produce a high magnification (×25) map. The        skilled man will appreciate that PSI mode is generally        appropriate for smoother surfaces where the height difference        between adjoining pixels does not lead to data loss. In        contrast, VSI mode is more suited to relatively rougher surfaces        in order to avoid such data loss. In the second step the        instrument revisits the defects identified by their        (x,y)-coordinates determined in the first step, in order to        yield more precise information about those regions of the film        sample in which major surface defects were located, and it is        the major intrinsic surface defects which are of primary        interest here. In the PSI mode, the cut-off threshold was set at        0.25 μm above the mean surface and the modulation threshold was        set at 10%. In the VSI mode, the cut-off threshold was also set        at 0.25 μm and the modulation threshold was set at 0.2%. In the        PSI mode, the relatively higher modulation threshold means that        the extrinsic defects can be inferred from “data-loss” regions.        Intrinsic defects were considered as those covering at least 9        adjoining but not necessarily co-linear pixels (1 pixel=0.35 μm)        and at least 0.25 μm in peak height. The values described herein        for N_(DT), N_(PP) and N_(GS) are derived from the PSI scans or        VSI scans, as appropriate, and particularly from the PSI scans,-   (xiii) Further surface analysis was conducted using XPS and static    SIMS spectroscopy, using a Kratos “Axis Ultra” instrument and an    lon-T of “ToFSIMS IV” instrument, respectively. The objective of the    analyses was to determine the presence of any residue from the    strippable sacrificial layer on the polyester substrate layer after    stripping of the strippable layer.    -   X-ray photoelectron spectroscopy (XPS) is a quantitative        spectroscopic technique which measures the elemental        composition, empirical formula, chemical and electronic states        of elements that exist within a material. XPS spectra are        obtained by using X-rays to irradiate a material while measuring        the kinetic energy and number of the emitted electrons from the        top 1 to 10 nanometers of the material. The detection limit is        around 1 atom in 1000 (excluding H, i.e. 0.1 atomic percent or        1000 ppm).    -   Secondary ion mass spectrometry (SIMS) is a technique involving        sputtering material surfaces with a primary ion beam and        collecting and analyzing emitted secondary ions. A mass        spectrometer is used to measure the secondary ions to determine        the elemental and/or molecular composition of the material        surface. Static SIMS is the process used in atomic monolayer        analysis of material surfaces, and has a typical sampling depth        of about 1 nm. SSIMS is not generally suitable as a quantitative        technique for individual analyses, but can usefully be used to        compare a series of similar sample surfaces via analysis of peak        area ratios as a measure of the relative amounts of identified        species present in those surfaces.    -   Freshly peeled surfaces were generated by cutting and tearing        the coated polyester substrate. Both sides of the peel were        analysed. Small pieces (ca. 1 cm×1 cm) were cut from the main        sample for analysis (ca. 10 cm×15 cm) using clean stainless        steel scissors. Samples for analysis were mounted on to suitable        sample holders using small pieces of silicone-free double-sided        tape. The samples for analysis were handled using clean        stainless steel tweezers at all times.    -   For the XPS analysis, data were recorded from a ca. 300 μm×700        μm elliptical area using monochromated Alka X-rays. A survey        scan was recorded at 160 eV pass energy to identify all elements        present on the surface; these were also used to quantify the        surface composition. High-resolution spectra were also recorded        at 20 eV pass energy in order to identify the chemical        environment for specific elements. The results are presented as        relative atomic percentage compositions. The expanded        uncertainty (Y) in atomic percentage units, associated with a        measured atomic percentage composition (X) was calculated for        polymer and organic materials analysed using survey scan        conditions from the expression Y=mX+c where m=0.027 and c=0.14.        The reported expanded uncertainty is based on a standard        uncertainty multiplied by a coverage factor of k=2, providing a        level of confidence of approximately 95%.    -   For the SSIMS analysis, positive and negative ion spectra were        recorded from fresh areas of each sample with high mass        resolution (m/Δm ca. 6000) up to m/z 2000 in all cases.-   (xiv) Melting temperature is determined by differential scanning    calorimetry (DSC) according to ASTM D3418.-   (xv) The VICAT softening point is determined by ASTM D1525.

The invention is further illustrated by the following examples. Theexamples are not intended to limit the invention as described above.Modification of detail may be made without departing from the scope ofthe invention.

EXAMPLES Examples 1 to 23

Attempts were made to manufacture a series of coextruded films having anAB layer structure. A polymer composition comprising unfilled PET wasco-extruded with a series of polymers (see Table 1), cast onto a cooledrotating drum, optionally pre-heated to a temperature of 80 to 81° C.and stretched in the direction of extrusion to approximately 3.4 timesits original dimensions. Where possible, the film was heated to atemperature of about 95° C., passed into a stenter oven at a temperatureof 110° C. where the film was stretched in the sideways direction toapproximately 3.6 times its original dimensions, and then thebiaxially-stretched film was heat-set by successive heating in threezones of defined temperature (225, 225 and 190° C.) by conventionalmeans at a film-web speed of 10.8 m/min; approximate residence time ineach of the three zones was 40 seconds. Table 1 characterises themanufacturing processes and the resulting films.

TABLE 1 Layer A MFI (test thickness Layer B (PET) Ex. Polymermethod)^(†) (μm) thickness (μm) Observations  1 EMAA  8   10 to 16 80-120 No major process issues. Biaxially oriented film manufacturedwith A (Nucrel ® 0908HS; DuPont ®) (a) Layer of even thickness. No MD orflow lines. Clean peel.  2 EMAA 11   10 to 16  80-120 No major processissues. Biaxially oriented film manufactured with A (Nucrel ® 0411HS;DuPont ®) (a) Layer of even thickness. No MD or flow lines. Clean peel. 3 Sodium ionomer of EMAA  2.5  10 to 16  80-120 Biaxially oriented filmmanufactured, but MD lines present in PET (Surlyn ® 1605; DuPont ®) (a)and Surlyn layers. Clean peel  4 Zinc ionomer of EMAA  5.2  10 to 16 80-120 Biaxially oriented film manufactured, but MD lines present inPET (Surlyn ® 1652; DuPont ®) (a) and Surlyn layers. Clean peel.  5Polypropylene  4.5  3 to 7  80-100 Biaxially oriented film manufactured,but Layer A residue remains on (DSM ®) (b) PET after peel.  6Polypropylene  3.5  40-50 500-600 Not possible to make a biaxiallyoriented film, due to partial peel-off (HKR 102; Sasol  ®) (b) of LayerA after the forward draw stage. Monoaxially oriented film onlymanufactured. Layer A exhibited flow lines and uneven layer thickness. 7 Polypropylene  1.8   70-100 250-350 Not possible to make a biaxiallyoriented film. Layer A peeled off at (PPH 3060; Total ®) (b) the forwarddraw stage. Cast film only manufactured.  8 Polypropylene copolymer 10  40-50 300-400 Monoaxially oriented film only made. Layer A exhibited MDlines. (PPR 7220; Total ®)) (b)  9 Polypropylene copolymer  7    6 to 14100-200 Biaxially oriented film made, although some inter-layer adhesionloss (PPC 5660; Total ®)) (b) during manufacture. Layer A exhibiteduneven thickness, flow lines, and high haze (>50%). Substrate exhibitedincreased haze and surface micro-roughness after removal of layer A. 10Polypropylene copolymer  1.8  55-65 200-400 Monoaxially oriented filmonly made. Layer A exhibited flow lines, (PPR 3260; Total ®)) (b) heavyMD lines and large thickness variation in transverse dimension. 11Polypropylene copolymer 16   55-65 200-400 Not possible to make abiaxially or monoaxially oriented film. Layer (CPV 340; Sasol ®) (b) Apeels off at cast film stage. 12 Polypropylene 21   55-65 200-400 Notpossible to make a biaxially or monoaxially oriented film. Layer (HRV140; Sasol ®) (b) A peels off at cast film stage. 13 Polypropylene 35  55-65 200-400 Not possible to make a biaxially or monoaxially orientedfilm. Layer (PPH 10042; Total ®)) (b) A peels off at cast film stage. 14Polypropylene copolymer 50   55-65 200-400 Not possible to make abiaxially or monoaxially oriented film. Layer (CTV 448; Sasol ®) (b) Apeels off at cast film stage. 15 Polyethylene  1   10-16 40-50 Biaxiallyoriented film made. Layer A exhibited uneven thickness. (HPs900-C;Nova ®) (a) Residue and remnants of Layer A remains on base layer afterpeel. 16 Polystyrene  2   120 400-500 Cast film only made. Not possibleto make a biaxially or monoaxially (Edistir 2380; Polimeri ®) (c)(brittle) oriented film. Layer A too brittle and peeled off at cast andforward draw stage. 17 Polystyrene  3.8  165-185 400-500 Cast film onlymade. Not possible to make a biaxially or monoaxially (Edistir 2560;Polimeri ®) (c) oriented film. Layer A peels off at forward draw stageor earlier. Layer A too brittle for clean peel. 18 Polystyrene  4   30Biaxially oriented film made. Layer A exhibits poor mechanical (EdistirHIPS R850E; Polimeri ®) (c) strength and fractures on peeling. 19Acrylonitrile butadiene styrene  1   Biaxially oriented film made. LayerA is brittle but does not peel (H1100 Natural; LG Chemical ®) (d) awayfrom base layer 20 Nylon 6,6  60-100 600-700 Not possible to make abiaxially or monoaxially oriented film. Layer (Vydyne 66B; Ascend ®) Aadheres to cast base layer but separates in forward draw 21 Ethylenevinyl acetate  9.5  500-600 Layer A adheres to cast base layer, but notmanufacture terminated (Appeel ® 11D554; DuPont ®) (a) due to fuming ofpolymer. 22 Ethylene vinyl acetate  2.65 N/A Biaxially oriented filmmade. Layer A not peelable from base layer. (Appeel ® 22D843; DuPont ®)(a) 23 Polyethylene naphthalate N/A Biaxially oriented film made. LayerA not peelable from base layer. (Teijin ®) ^(†)The test methods used forthe Melt Flow Index of the polymer of Layer A were: (a) ASTM D1238;using a temperature of 190° C. and a mass of 2.16 kg (b) ISO 1133; usinga temperature of 230° C. and a mass of 2.16 kg (c) ISO 1133; using atemperature of 200° C. and a mass of 5 kg (d) ASTM D1238; using atemperature of 200° C. and a mass of 5 kg

Where it was possible to manufacture a biaxially oriented composite filmwhich exhibited adequate mechanical strength and little or nointer-layer delamination during manufacture, the film was analysed usingthe hand-peel test described herein. In this test, Examples 1 to 4 and 9satisfied grade 1; Example 5 satisfied grade 2, while the remainingExamples either were categorised as grade 3 or 4 or were unable to bemanufactured as a biaxially oriented composite film. The peel force ofExamples 1 to 5 and 9 was then measured according to the methoddescribed herein, and the results are set out in Table 2 below.

The haze, TLT and Ra of the polyester substrate immediately afterpeeling were examined for Examples 1 to 5 and 9, and the results arereported in Table 2 below, along with a control film consisting of themono-layer PET film corresponding to the substrate of the Examples. Thehaze values of Examples 1 to 5 were excellent, whereas the haze forExample 9 was significantly worse. Similarly, the Ra values for surfacemicro-roughness of Examples 1 to 5 were all much lower than that ofExample 9, indicating that the polypropylene strippable layer haddisadvantageously imparted some additional texture or roughness or otherdefect during manufacture or storage of the composite film, or duringthe act of stripping.

The presence of scratches on the film surface was assessed by opticalmicroscopy, as described herein. The surface of the control filmexhibited a significant number of scratches, whereas the freshly peeledsurfaces of Examples 1 to 5 and 9 exhibited none.

TABLE 2 Hand-peel test Peel Force TLT Haze Ra Example (grade)(grams-Force) (%) (%) (nm) Control — — 84.6 0.47 0.98 1 1 202 84.9 1.510.8  2 1 63.8 86.6 1.09 4.01 3 1 176 85.2 0.64 3.31 4 1 60 85.4 0.811.33 5 2 29 85.5 1.5  — 9 1 5.3 85.3 9.55 81.4

The experiments demonstrate that the EMAA strippable layers uniquely andunexpectedly show the required combination of properties.

The films of Examples 1 and 2 were also analysed by the XPS and SSIMStechniques described hereinabove in order to determine the presence ofany chemical residue on the substrate immediately after peeling away thestrippable layer. In both analyses, no evidence for any chemical residueof the strippable layer was found on the PET substrate for eitherExample 1 or Example 2.

In the XPS analyses, the chemical composition (measured as relativeatomic percentage composition) and high-resolution spectra of thesurface of the freshly peeled PET substrate corresponded to thosetypically observed for PET film. Thus, the measured level for therelative atomic percentage of carbon of the freshly peeled surface fellwithin the typical range for PET surfaces of 72.0-76.4 atomic %. The C1sand O1s high resolution spectra from the surface of the freshly peeledPET substrate were also typical of clean PET, showing C—C, C—O, O—C═O inthe expected proportions.

In the SSIMS analyses, the spectra recorded from the surface of thefreshly peeled PET substrate were consistent with the presence of apristine PET layer.

Examples 24 to 26

The EMAA-coated examples from the first set of experiments wereinvestigated further, in particular by measuring the intrinsic surfaceroughness by the LAM technique described hereinabove. Example 25(Nucrel®0908HS) and Example 26 (Nucrel®0411HS) are coextruded filmsmanufactured according to Examples 1 and 2. Example 24 is a controlexample which consists of a coextruded bi-layer PET film in which bothlayers have the same composition as the substrate layer in Example 1,and otherwise processed in the same way as Example 1. The totalthickness, and each layer thickness, of Example 24 were the same asthose in coextruded films of Example 25 and 26, in order to providesimilarity in processing conditions. Thus, the film of Example 24 ismade up of a thin PET layer A and a thick PET layer B. The surfaces ofthe polyester substrate in Examples 25 and 26 were analysed directlyafter stripping the sacrificial EMAA layer by hand. The surface of thethin PET layer A of the polyester substrate of control Example 24 wasanalysed directly after manufacture. The results are presented in Table3 below. The values for N_(DT), N_(PP) and N_(GS) are derived from PSIscans. The values for Δ-N_(DT), Δ-N_(PP) and Δ-N_(GS) are calculated forExamples 25 and 26 using Example 24 as the control film. All threeexamples were manufactured on a standard film-forming line and nospecial steps were taken to provide to a clean environment or to reducethe amount of air-borne dust and debris.

TABLE 3 Ex. N_(DT) Δ-N_(DT) N_(PP) Δ-N_(PP) N_(GS) Δ-N_(GS) N_(E) Ex. 242796 — 366 — 43 — 76 Ex. 25 718 3.9 63 5.8 0 → ∞ 13 Ex. 26 443 6.3 546.8 0 → ∞ 36

The data in Table 3 demonstrate that not only does the strippable layerprotect the polyester substrate from extrinsic debris (and the damagethat results from such extrinsic debris), it also unexpectedly improves(i.e. decreases) the intrinsic surface roughness of the substrate.

Examples 27 and 28

The EMAA-coated examples were investigated further using the LAMtechnique. Example 27 is a further control example and consists of amonolayer of unfilled PET of the same composition as the PET layers inthe coextruded bi-layer film of (control) Example 24. Example 28 is acoextruded film corresponding to Example 26, comprising a substratelayer of the same unfilled PET of Example 27, and a strippablesacrificial layer of Nucrel®0411HS. The PET layer in each of Examples 27and 28 is the same thickness and is derived from the same (primary)extruder. The surface of the PET substrate in Example 28 was analysed asbefore and directly after stripping the sacrificial layer by hand, andthen compared with the control surface of the PET monolayer film ofExample 27 which was analysed directly after manufacture. The resultsare presented in Table 4 below. The absolute defect numbers in Table 4are lower than in Table 3 because additional steps were taken duringfilm manufacture to provide a clean environment. Nevertheless, the dataconfirm the unexpected result observed from the comparison of Examples24 and 26.

TABLE 4 Ex. N_(DT) Δ-N_(DT) N_(PP) Δ-N_(PP) N_(GS) Δ-N_(GS) N_(E) Ex. 2794 — 22 — 33 — 36 Ex. 28 81 1.16 4 5.5 1 33 6

The invention claimed is:
 1. A coextruded biaxially oriented compositefilm comprising a polyethylene terephthalate substrate layer anddisposed on one or both surfaces thereof a strippable sacrificial layer,wherein said strippable sacrificial layer consists of anethylene-methacrylic acid (EMAA) copolymer, and wherein said one or bothsurfaces exhibit(s) an Ra of less than 10 nm upon removal of thestrippable sacrificial layer(s), wherein the strippable sacrificiallayer has a thickness in the range of from about 2 to about 200 μm andthe ethylene-methacrylic acid (EMAA) copolymer has a melt flow index of0.5 to 50 g/10 min, measured according to ASTM D1238.
 2. The filmaccording to claim 1 wherein the methacrylic acid is present in the EMAAcopolymer in the range of from about 2 to about 15 wt % of thecopolymer.
 3. The film according to claim 1 wherein the EMAA copolymeris an ionomer comprising a minor proportion of metal salt-containingunits of methacrylic acid.
 4. The film according to claim 3 wherein themetal is selected from alkali metals, magnesium and zinc.
 5. The filmaccording to claim 1 wherein the EMAA copolymer is an ionomer selectedfrom copolymers of ethylene and methacrylic acid partially or completelyneutralised with metal cation(s).
 6. The film according to claim 5wherein the metal is selected from alkali metals, magnesium and zinc. 7.The film according to claim 1 wherein the thickness of the substratelayer is in the range of from about 5 to about 500 μm and/or thethickness of the strippable layer is in the range of from about 2 toabout 100 μm.
 8. The film according to claim 1 wherein the adhesivestrength of the strippable sacrificial layer to the polyethyleneterephthalate substrate layer is such that the peel force is in therange of from about 20 to about 250 gF.
 9. The film according to claim 1wherein the polyethylene terephthalate substrate layer has a haze of nomore than 15% and/or a total luminous transmission (TLT) for light inthe visible region (400 nm to 700 nm) of at least 80%.
 10. The filmaccording to claim 1 wherein the polyethylene terephthalate substratelayer exhibits a shrinkage of no more than 3% at 150° C. over 30minutes.
 11. A method of protecting a surface of a substrate from damageand/or contamination and/or debris during transport and/or storage, saidmethod comprising the steps of: (i) providing a coextruded compositefilm comprising a polyethylene terephthalate substrate layer anddisposed on one or both surfaces thereof a strippable sacrificial layeraccording to claim 1; and (ii) removing said strippable sacrificiallayer from said substrate prior to use or further processing of saidsubstrate.
 12. The method according to claim 11 wherein the wherein theethylene-methacrylic acid (EMAA) copolymer consists of units of ethyleneand units of one or both of methacrylic acid and metal salt-containingunits of methacrylic acid.
 13. The method according to claim 11 whereinthe EMAA copolymer is not an ionomer and does not contain metal ions.14. A method of reducing defects in a functional layer applied to asubstrate, said method comprising the steps of: (i) providing acoextruded composite film comprising a polyethylene terephthalatesubstrate layer and disposed on one or both surfaces thereof astrippable sacrificial layer according to claim 1; (ii) removing saidstrippable sacrificial layer from said substrate; and (iii) applyingsaid functional layer to said substrate.
 15. The method according toclaim 14 wherein said functional layer is a conductive layer or barrierlayer.
 16. The method according to claim 14 wherein theethylene-methacrylic acid (EMAA) copolymer consists of units of ethyleneand units of one or both of methacrylic acid and metal salt-containingunits of methacrylic acid.
 17. The method according to claim 14 whereinthe EMAA copolymer is not an ionomer and does not contain metal ions.18. An electronic or opto-electronic device, optical film, medicaldevice or decorative film derived from a coextruded biaxially orientedcomposite film as defined in claim 1 from which said strippablesacrificial layer(s) has/have been removed, wherein said one or bothsurfaces exhibit(s) an Ra of less than 10 nm upon removal of thestrippable sacrificial layer(s).
 19. The electronic or opto-electronicdevice according to claim 18 comprising a polyethylene terephthalatesubstrate layer derived from the coextruded composite film comprisingsaid polyethylene terephthalate substrate layer and disposed on one orboth surfaces thereof a strippable sacrificial layer, wherein saidstrippable sacrificial layer is removed from said composite film priorto or during incorporation into or manufacture of said electronic oropto-electronic device.
 20. The electronic or opto-electronic deviceaccording to claim 18, further comprising a barrier or conductive layer,wherein said barrier or conductive layer is disposed on saidpolyethylene terephthalate substrate layer in said electronic oropto-electronic device.
 21. The electronic or opto-electronic deviceaccording to claim 18, selected from electroluminescent (EL) displaydevices, electrophoretic display devices, photovoltaic cells andsemiconductor devices.
 22. The electronic or opto-electronic device,optical film, medical device or decorative film according to claim 18wherein the ethylene-methacrylic acid (EMAA) copolymer consists of unitsof ethylene and units of one or both of methacrylic acid and metalsalt-containing units of methacrylic acid.
 23. The electronic oropto-electronic device, optical film, medical device or decorative filmaccording to claim 18 wherein the EMAA copolymer is not an ionomer anddoes not contain metal ions.
 24. The device according to claim 18,wherein said one or both surfaces exhibit(s) a number (N_(GS)) ofgel-like defects with a peak height of greater than about 0.25 μm andless than about 30 μm of no more than 10 per 31×33 cm area of filmsurface upon removal of the strippable sacrificial layer, and/or whereinthe improvement (Δ-N_(GS)) in the parameter (N_(GS)) relative to acontrol polyester substrate manufactured without the strippablesacrificial layer is at least 2, wherein Δ-N_(GS) is defined as: [N_(GS)of said control polyester substrate]/[(N_(GS)) of surface of saidpolyester substrate layer upon removal of said strippable sacrificiallayer].
 25. The device according to claim 18, wherein said one or bothsurfaces exhibit(s) a number (N_(DT)) of defects with a verticalamplitude, orthogonal to the film plane, of greater than about 0.25 μmand less than about 30 μm above and/or below the mean surface of no morethan 1000 per 31×33 cm area of film surface upon removal of thestrippable sacrificial layer, and/or wherein the improvement (Δ-N_(DT))in the parameter (N_(DT)), relative to a control polyester substratemanufactured without the strippable sacrificial layer, is at least 2,wherein Δ-N_(DT) is defined as: [N_(DT) of said control polyestersubstrate]/[(N_(DT)) of surface of said polyester substrate layer uponremoval of said strippable sacrificial layer].
 26. The device accordingto claim 18, wherein said one or both surfaces exhibit(s) a number(N_(PP)) of pinch-point peaks with a peak height of greater than about0.25 μm and less than about 30 μm of no more than 100 per 31×33 cm areaof film surface upon removal of the strippable sacrificial layer, and/orwherein the improvement (Δ-N_(PP)) in the parameter (N_(PP)), relativeto a control polyester substrate manufactured without the strippablesacrificial layer, is at least 2, wherein Δ-N_(pp) is defined as:[N_(PP) of said control polyester substrate]/[(N_(PP)) of surface ofsaid polyester substrate layer upon removal of said strippablesacrificial layer].
 27. A method of using a layer consisting of anethylene-methacrylic acid (EMAA) copolymer as a strippable sacrificiallayer in a coextruded biaxially oriented composite film furthercomprising a polyethylene terephthalate substrate layer, as defined inclaim 1, wherein said EMAA layer is disposed on one or both surfaces ofsaid polyethylene terephthalate substrate layer, said method comprisingstripping the layer consisting of the ethylene-methacrylic acid (EMAA)copolymer from the polyethylene terephthalate substrate layer.
 28. Themethod according to claim 27 wherein the ethylene-methacrylic acid(EMAA) copolymer consists of units of ethylene and units of one or bothof methacrylic acid and metal salt-containing units of methacrylic acid.29. The method according to claim 27 wherein the EMAA copolymer is notan ionomer and does not contain metal ions.
 30. The film according toclaim 1 wherein the ethylene-methacrylic acid (EMAA) copolymer consistsof units of ethylene and units of one or both of methacrylic acid andmetal salt-containing units of methacrylic acid.
 31. The film accordingto claim 1 wherein the EMAA copolymer is not an ionomer and does notcontain metal ions.
 32. The film according to claim 1, wherein said oneor both surfaces exhibit(s) a number (N_(GS)) of gel-like defects with apeak height of greater than about 0.25 μm and less than about 30 μm ofno more than 10 per 31×33 cm area of film surface upon removal of thestrippable sacrificial layer, and/or wherein the improvement (Δ-N_(GS))in the parameter (N_(GS)), relative to a control polyester substratemanufactured without the strippable sacrificial layer, is at least 2,wherein Δ-N_(GS) is defined as: [N_(GS) of said control polyestersubstrate]/[(N_(GS)) of surface of said polyester substrate layer uponremoval of said strippable sacrificial layer].
 33. The film according toclaim 1, wherein said one or both surfaces exhibit(s) a number (N_(DT))of defects with a vertical amplitude, orthogonal to the film plane, ofgreater than about 0.25 μm and less than about 30 μm above and/or belowthe mean surface of no more than 1000 per 31×33 cm area of film surfaceupon removal of the strippable sacrificial layer, and/or wherein theimprovement (Δ-N_(DT)) in the parameter (N_(DT)), relative to a controlpolyester substrate manufactured without the strippable sacrificiallayer, is at least 2, wherein Δ-N_(DT) is defined as: [N_(DT) of saidcontrol polyester substrate]/[(N_(DT)) of surface of said polyestersubstrate layer upon removal of said strippable sacrificial layer]. 34.The film according to claim 1, wherein said one or both surfacesexhibit(s) a number (N_(PP)) of pinch-point peaks with a peak height ofgreater than about 0.25 μm and less than about 30 μm of no more than 100per 31×33 cm area of film surface upon removal of the strippablesacrificial layer, and/or wherein the improvement (Δ-N_(PP)) in theparameter (N_(PP)), relative to a control polyester substratemanufactured without the strippable sacrificial layer, is at least 2,wherein Δ-N_(PP) is defined as: [N_(PP) of said control polyestersubstrate]/[(N_(PP)) of surface of said polyester substrate layer uponremoval of said strippable sacrificial layer].